Lubricating a tribological system

ABSTRACT

A method and gas lubricant are provided for lubricating a tribological system and include providing gaseous carbon dioxide, and bringing the gaseous carbon dioxide into contact with regions of the tribological system to be lubricated. An inert gas may also contact the regions to reduce oxidation. Nitrogen may be used as the inert gas.

The invention relates to a method and a device for lubricating atribological system.

Tribological systems comprise at least two parts, which perform arelative movement to one another while in contact, thereby creatingfriction and wear. Such friction and wear created by the movement andthe contact of such parts can result in a deterioration of the systemand a shorter lifetime.

For example, heavy load bearings for machine tools, gear boxes, turbines(gas turbines) or space equipment are subject to wear and friction,which must be minimized in order to ensure longevity.

To this effect, special bulk materials, surfaces and/or lubricants areconstantly being developed, in order to decrease friction and wear,thereby increasing the lifetime of such systems.

As lubricants, it is well known to use oil-based substances. Theatmosphere surrounding such tribological systems is usually air.However, air is not inert and reacts with the surfaces or bulk materialof the components of a tribological system (for example metals) and alsowith lubricants. Especially, air can lead to deterioration due tooxidation of metals and/or lubricants. These disadvantages areespecially prominent in case of high temperatures.

Such interactions between the tribological system and the surroundingatmosphere have an important overall effect on the lifetime of thesystem as well as its efficiency.

From WO 2006/119047 A2 it is known to use supercritical carbon dioxideas a lubrication means for metal work pieces during metal workingprocesses. The provision of supercritical carbon dioxide is, however,relatively expensive. Also, the provision and handling of supercriticalcarbon dioxide is time-consuming.

The present invention thus seeks to provide a lubricant for tribologicalsystems which is efficient, as well as easy and cheap to provide andhandle.

The invention thus provides a method according to the independentclaims.

Advantageous embodiments of the invention are the subject matter of thedependent claims.

According to the invention, a highly effective lubricant for numeroustribological systems is provided, which is both inexpensive and easy tohandle.

It has been found that carbon dioxide reduces friction and wear.Therefore, the invention proposes to bring gaseous carbon dioxide incontact with the regions of the tribological system which shall belubricated. However, it has also been found that high carbon dioxideconcentrations at the tribological system can cause the formation ofmetal oxides at the surface of the tribological system, such as a gearor a bearing. Furthermore, high carbon dioxide formation can also causeoxidation of the lubricant. Therefore, according to the inventiongaseous carbon dioxide and an inert gas are provided in order to avoidtoo high carbon dioxide concentrations. Compared to air, inert gaseswill reduce friction and due to their inertness oxidation of thelubricant and the system to be lubricated will be prevented.

The gaseous carbon dioxide and the inert gas can be provided as amixture or as separate gas feeds. In any case the carbon dioxide isdiluted with the inert gas in order to avoid too high carbon dioxideconcentrations at the tribological system.

In addition to its inertness, nitrogen has shown to reduce wear comparedto air. Therefore, using nitrogen as inert gas in the inventive processwill improve the friction and wear behavior of the tribological systemand reduce oxidation of the metal surfaces of the tribological systemand of the lubricant and its components.

The amount and concentration of carbon dioxide at the tribologicalsystem, e.g. a gear or a bearing, is preferably controlled to be suchthat a tribological layer is formed on the surfaces of the tribologicalsystem but oxidation of the system and the lubricant is prevented.

It is preferred to analyze the lubricant and/or to monitor a parametercharacterizing the operation of the tribological system and to controlthe supply of carbon dioxide and/or inert gas based on these data. Forexample, when friction increases during operation of a tribologicalsystem this has an influence on the required energy input, the poweroutput and/or the speed of the system. Monitoring these parametersprovides information on the friction. A reduction of speed or anincrease of the power and energy input to a gear indicate an increasedfriction. In that case the amount of carbon dioxide and/or the ratio ofcarbon dioxide to inert gas are increased to improve the lubricatingbehavior.

The analysis of the lubricant can also be an indicator for the status ofthe tribological system. The supply of carbon dioxide and/or inert gasto the tribological system is preferably controlled based on an analysisof the lubricant. At too high carbon dioxide levels or at too low inertgas levels the lubricant or components of the lubricant can be oxidized.When the analysis of the lubricant shows the formation of oxides theamount, pressure and/or gas flow of carbon dioxide provided to thetribological system will be reduced and/or the amount, pressure and/orgas flow of inert gas will be increased.

For example part of the lubricant can be continuously passed through ananalyzer which determines the presence of oxides in the lubricant.

According to a preferred embodiment, gaseous carbon dioxide is providedby expanding liquid carbon dioxide provided in a gas cylinder or vessel.It can be transformed into the gaseous state for example using membranetechniques. After being directed to the tribological system to belubricated (i.e. regions of the tribological system, which are to belubricated), this gaseous carbon dioxide can be released into theatmosphere. As is well known, health hazards in connection with carbondioxide are minimal, so that no specific measures for protecting usersin the vicinity of the lubricated tribological system are necessary.Gaseous carbon dioxide can be supplied from cylinders with gaseouscontent. It is also possible to collect carbon dioxide after lubricationin order to recycle it, for example to provide synthetic fuels.

Gaseous carbon dioxide can easily be handled. For example, it can beprovided in cylinders or containers containing liquid carbon dioxide.Such a cylinder can be provided with means for gaseous extraction, as iscommonly known. Carbon dioxide is cheap and readily available.

The invention will now be described in more detail on the basis of apreferred embodiment as shown in the FIGURE.

The invention has several advantages compared to the prior art. Carbondioxide has shown to reduce friction and wear of a tribological system.However, when the tribological system is lubricated by a lubricant, suchas oil, the lubricant can be oxidized by excess carbon dioxide.Therefore, the invention proposes to use a mixture of carbon dioxide andinert gas as the atmosphere surrounding the tribological system and thelubricant.

Nitrogen shows a wear reducing behavior as well as it avoids oxidationof the lubricant and of parts of the tribological system. Therefore, amixture of carbon dioxide and nitrogen is advantageous.

By controlling the operation and/or efficiency of the tribologicalsystem or characteristic parameters of the tribological system, such asenergy input and output, power input and output, or speed of movingparts of the tribological system, the actual status of friction can bedetermined and the provision of carbon dioxide and/or inert gas can becontrolled accordingly. The analysis of the lubricant or its componentscan be another parameter indicating the effectiveness of thelubrication. High oxide concentrations in the lubricant indicate toohigh carbon dioxide levels. In that case the carbon dioxide flow to thetribological system is reduced and/or the ratio of carbon dioxide toinert gas is reduced.

FIG. 1 shows a preferred embodiment of a device according to theinvention with which a tribological system can be lubricated.

As an example of a tribological system to be lubricated, a bearing 10 isshown. A cylinder 20 containing liquid carbon dioxide is provided with anozzle device 22 (not shown true to scale relative to the cylinder 20),through which liquid carbon dioxide can be expanded to provide a stream24 of gaseous carbon dioxide. A closing valve 21 is expediently providedbetween cylinder 20 and nozzle device 22.

By expedient placement of nozzle 22 in relation to the bearing 10,lubrication of the latter by means of gaseous carbon dioxide can beeffectively provided.

As carbon dioxide is non-hazardous for users, no means for retrievinggaseous carbon dioxide subsequently to its contact with bearing 10 arenecessary.

Injection of gaseous carbon dioxide enables an effective lubrication ofregions of tribological systems to be lubricated. Especially, specificshapes of nozzles 22 can be used in order to transport gaseous carbondioxide to desired regions or places.

Wear and friction of tribological systems, especially metal parts ofsuch systems, is thus effectively reduced.

This observation especially holds for tribological systems operating atelevated temperatures, especially for temperatures above 200° C.,

It is presumed that by usage of gaseous carbon dioxide as lubricantmetal or iron carbonates are formed on the surface of components oftribological systems, resulting in efficient protective layers.

A continuous supply of gaseous carbon dioxide is usually necessary,since such protective layers of metal or ion carbonates will be removedby rubbing, i. e. components of a tribological system moving relative toone another.

It has been shown that usage of gaseous carbon dioxide as lubricantrequires no further addition of other lubricants, such as oil basedlubricants.

The invention constitutes a dry lubrication, which can especially beutilized for applications in which any kind of dampness is to beavoided.

In most tribological applications, liquid or grease lubricants are usedagainst friction and wear. However, under extreme conditions e. g. (veryhigh or low temperature, vacuum, radiation, extreme pressure) solidlubricants (i.e. molybdenum disulfide, graphite, hexagonal boronnitride) have conventionally been the only choice. At least for some ofthese applications, gaseous carbon dioxide may now be used as lubricant.

Examples of applications are: air compressors, food industryapplications, railway track joints, open gear, ball bearings,machine-shop works, space vehicles (bearings, rings).

The invention is also useful to reduce wear and friction in tribologicalsystems containing materials such as ceramics and polymers.

Further preferred examples of tribological systems, with which theinvention can advantageously be used, are, for example, bearings,magnetic storage devices, transmission devices, driveline devices,brakes, cylinders, valves, wheels, dampers, equipment used in mining andmineral processing, cutting devices, gearboxes, turbines and spaceequipment. Spacecraft contain a variety of instruments and mechanismsthat require lubrication. Such devices include solar array drives,momentum, reaction and filter wheels, tracking antennas, scanningdevices and sensors.

What is claimed is:
 1. A method for lubricating a tribological system, comprising: providing gaseous carbon dioxide; contacting the gaseous carbon dioxide with regions of the tribological system to be lubricated; and providing an inert gas for contacting the regions of the tribological system to be lubricated.
 2. The method of claim 1, wherein the providing the gaseous carbon dioxide is by expanding liquid carbon dioxide.
 3. The method of claim 1, wherein said gaseous carbon dioxide and said inert gas are provided as a gas mixture.
 4. The method of claim 1, wherein said inert gas comprises nitrogen.
 5. The method of claim 1, wherein the gaseous carbon dioxide and the inert gas are provided as a gaseous mixture brought into contact with the regions of the tribological system at a pressure between 0.1 MPa and 0.5 MPa.
 6. The method of claim 1, further comprising monitoring a parameter of operation of the tribological system, and controlling the providing of at least one of the gaseous carbon dioxide and the inert gas based on the parameter monitored.
 7. The method of claim 6, wherein the parameter monitored is selected from the group consisting of energy input, energy output, power input, power output, and a speed of a moving part of the tribological system.
 8. The method of claim 1, wherein the tribological system operates during contact with a lubricant.
 9. The method of claim 8, further comprising analyzing a composition of a lubricant resulting from the gaseous carbon dioxide and the inert gas, and controlling the providing of at least one of the gaseous carbon dioxide and the inert gas based on an analysis of the lubricant.
 10. Using a mixture of gaseous carbon dioxide and an inert gas as a lubricant for a tribological system.
 11. The use of the mixture of claim 10, wherein the inert gas comprises nitrogen.
 12. The use of the mixture of claim 10, further comprising providing the gaseous carbon dioxide and the inert gas as a gas mixture for contacting the tribological system.
 13. A gas lubricant for a tribological system, comprising: gaseous carbon dioxide; and an inert gas.
 14. The gas lubricant of claim 13, wherein the gaseous carbon dioxide and the inert gas are a gas mixture.
 15. The gas lubricant of claim 13, wherein the inert gas comprises nitrogen. 